A hydrodynamic bearing system essentially includes a bearing sleeve, a shaft accommodated in an inner cylindrical bore of the bearing sleeve and at least one radial bearing section provided between the bearing sleeve and the shaft with the aid of which the shaft and the bearing sleeve are supported rotatably with respect to each other. A bearing gap is formed between the shaft and the bearing sleeve. The bearing gap is filled with a liquid lubricant, preferably bearing oil.
To take on axial loads, the bearing system is also provided with a hydrodynamic thrust bearing. The thrust bearing is formed by a thrust plate arranged at one end of the shaft and a corresponding cover plate. The cover plate forms a counter bearing to the thrust plate and seals the entire bearing system from below so that no lubricant can escape from the bearing system.
In many cases, the connection between the thrust plate and the shaft is realized by means of a press connection. With motors and hard disk drives becoming ever smaller in size, the overall length available for the bearing system is also being reduced. One method which attempts to overcome this situation involves reducing the thickness of the thrust plate. To achieve an optimal press connection, the so called guiding ratio, the quotient of the compression length t and bore diameter d, should be greater than or equal to 1.
The thinner the thrust plate, the harder it is to achieve the required perpendicularity and the greater the excess size of the shaft in relation to the bore has to be in order to achieve the specified press-out force. This increases the risk that on being mounted onto the shaft, the thrust plate adheres to the shaft resulting in destroying the perpendicularity between the shaft and the thrust plate.
To avoid this problem, when using very thin thrust plates, it is known to connect the thrust plate to the shaft by means of welding. This connecting method is revealed in JP 2000-324753. There is, however, the disadvantage and risk that the bearing system could become contaminated through welding residue which could cause damage to the bearing system. Due to the heat generated in the welding process, there is the added risk that the thrust plate could be deformed and thus rendered unusable.
Another possibility disclosed in U.S. Pat. No. 5,357,163 is to screw the thrust plate to the end face of the shaft. However, on one hand this means that a planar end face has to be provided at a right angle to the shaft end on the other hand this method requires an additional, fault-prone assembly effort.
Another possible solution is to form the thrust plate and the shaft as one piece. Manufacturing such an integral component with the required tolerances, however, involves a very complex and expensive process.
Published patent documents DE 19637014 A1 and DE 19637015 A1 disclose arrangements to mount machine parts on shafts, wherein the shaft formed as a sleeve is provided at one end with a collar set radially outwardly with a flange being force fitted to the collar end facing the shaft. Since the outer diameter of the collar is greater than the inner diameter of the flange bore, the flange has always to be mounted from the longer end of the shaft which is not possible for a hydrodynamic bearing system consisting of a radial and an axial bearing without the risk of damaging the radial bearing surface.
Patent document GB 274 954 relates to an arrangement affixing a machine part to one end of a shaft, wherein the shaft features an axial bore in the area where the machine part is to be positioned and an element to fix the machine part is inserted into this bore. The fixing element takes the form of a ball or a cylindrical plug whose outer diameter is greater than the inner diameter of the bore. Other embodiments of the fixing element are not described.